Lead-acid batteries are a well known source of energy used in a variety of applications including, for example, automotive starting. The central structural elements of conventional lead-acid batteries are positive and negative grids coated with active material to form plates, each having a lug and separated from each other within a battery by porous separators. The grids serve as framework and electrical contact to the positive and negative active materials which generally serve to conduct current. This conjoint electrochemical (corrosion) action and concurrent structural (load-bearing) role cause stress to the grids, particularly the positive grid. Failure of the battery occurs when the grids are no longer able to provide adequate structural support or allow electron flow. Therefore, the primary properties of interest in grid formation and design are strength, resistance to corrosion and mechanical stresses. Other properties to consider include castability, compatibility with the active material (adherence), and electrochemical and metallergic effects. With respect to the latter properties, fluidity and resistance to "hot-tearing" (shrinkage tearing) on solidification and grain formation generally are important.
Traditionally, grids have been constructed to be as constraining as possible. This design resists the dynamic effect of active material shape change and dimensional change in the surface of the grid due to corrosion. This is done by maximizing the cross sectional area of wires and using high tensile alloy materials that are environmentally compatible in a cell. This design invariably becomes a compromise between practical amounts of grid and active material and manufacturing constraints. Typically, the design is such that electron flow is directed in the shortest path to the collecting point or lug. Optimal sized pellets are selected and surrounded with grid material in a pattern to provide the necessary power requirement, and the matrix is thus formed and then framed to facilitate processing.
One method to form such a battery grid is termed book casting. In practicing book casting, a grid is formed in a mold made of two halves, wherein one-half of the mold remains stationary while the other half moves past for ejection of the grid. This process is undesirable because it requires substantial human interplay and one man typically operates 3 molds which produce a total of 60 grids a minute 80% of the time.
Methods have been developed to overcome the undesirableness of book casting such as a method termed expanded metal. This method is illustrated in U.S. Pat. Nos. 3,853,626; 4,247,970; and 4,271,586. Generally, a basic strip of lead typically 0.040 inches thick and 3 inches wide is lanced to form a skeleton material, structurally very weak. Without side frames the material is very difficult to process. The manufacturing rates utilizing this process have been known to reach 200 to 300 grids a minute. However, this method is undesirable due to the costliness of having to form the required grid strip prior to expansion. Additionally, this method does not lend itself to versatility in grid design and grids must be framed to process.
Another more desirable method to cast grids is termed continuous casting (con-casting), disclosed in U.S. Pat. No. 4,349,067. This method allows grids to be cast continuously and directly thus eliminating the need for any mechanical manipulation such as expansion. The grids can, for example, be formed from ingot lead into continuous strips of grids at rates of 200 to 300 grids a minute. The current process is undesirable however to form positive grids because of metallurgic effects resulting in improper grain formation leading to increased corrosion and mechanical stress on the grid. Positive grids differ from negative grids generally in that positive grids require greater strength due to anodic attack, and thus are generally formed with an increased cross-section width in comparison to negative grids. However, increased mold depth is not conducive to con-casting because grain formation is disrupted due to preferential cooling and turbulence caused by the con-casting machine. Imperfect grain formation is undesirable because preferred grain boundary attack leads to non-uniform corrosion. Corrosion and mechanical stresses lead to a swelling effect in the material due to growth of intergranular corrosion products, and give rise to apparent "growth" of grids in service causing the grid to distort until it becomes dysfunctional. Similarly, positive grids formed from metal expansion suffer such undesirable metallurgic properties. Additionally, current con-casting methods utilize wires having a trapezoidal shape which also contribute to poor performance and economical disadvantages of positive grids formed by current con-casting methods.
It would thus be desirable to provide a battery grid which can relieve stress at predetermined locations. It would further be desirable to provide a positive battery grid with a growth relief mechanism to accommodate the condition of imperfect grain formation without the interruption of the reticulum and structured such that it is easy to process. It would further be desirable to provide such a battery grid produced quickly, economically and efficiently.